Graded anti-reflective coating for IC lithography

ABSTRACT

A substantially continuously graded composition silicon oxycarbide (SiOC) antireflective coating (ARC) or antireflective layer (ARL) is interposed between a photoresist layer and an underlying substrate. The ARC matches an optical impedance at the interface between the ARC and photoresist. The optical impedance decreases (absorptivity increases) substantially continuously, in the ARC in a direction away from the interface between the ARC and the photoresist. The ARC composition is graded from SiOC, at its interface with the photoresist, to SiC or Si, in a direction away from the photoresist. Reflections at the ARC-photoresist interface are substantially eliminated. Substantially all incident light, including ultraviolet (UV) and deep ultraviolet (DUV) light, is absorbed in the ARC. As a result, substantially no light reaches or is reflected from the underlying substrate. Photolithographic limitations such as swing effect and reflective notching are reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending, commonly assigned patentapplication of Forbes et al., U.S. patent application Ser. No.08/903,486 entitled "SILICON CARBIDE GATE TRANSISTOR AND FABRICATIONPROCESS", filed on Jul. 29, 1997, which disclosure is incorporatedherein by reference.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to integrated circuits, andparticularly, but not by way of limitation, to a graded siliconoxycarbide anti-reflective coating for integrated circuitphotolithography.

BACKGROUND OF THE INVENTION

Trends in modem integrated circuit (IC) technology demand increasinglydense ICs, such as for computer systems, portable electronics, andtelecommunications products. IC fabrication includes, among otherthings, photolithography for selective patterning and etching ofphotoresist layers. The patterned photoresist layer serves as a maskinglayer such that a subsequent IC processing step is carried out on onlythose portions of the underlying IC that are uncovered by photoresist,as described below.

A photoresist layer is typically formed on an underlying integratedcircuit substrate. The photoresist layer overlays any structures thatare already formed on the substrate. Portions of the photoresist areselectively exposed to light through a lithographic mask that includesclear and opaque portions forming a desired pattern. Light istransmitted through the clear portions of the mask, but not through theopaque portions. The incident light changes the chemical structure ofthe exposed portions of photoresist. A chemical etchant, which issensitive to only one of the exposed and unexposed portions of thephotoresist, is applied to the photoresist to selectively remove thoseportions of the photoresist to which the chemical etchant is sensitive.As a result, portions of the photoresist which are insensitive to thechemical etchant remain on the IC. The remaining portions of thephotoresist protect corresponding underlying portions of the IC from asubsequent IC processing step. After this IC processing step, theremaining portions of the photoresist layer are typically removed fromthe IC.

High density ICs require sharply defmed photoresist patterns, becausethese patterns are used to define the locations (and density) ofstructures formed on the IC. However, light reflects from the surface ofthe underlying substrate on which the photoresist is formed. Certainstructures that are formed on the underlying substrate are highlyreflective such as, for example, aluminum layers for forming circuitinterconnections. Reflections from the surface of the substrateunderlying the photoresist causes deleterious effects that limit theresolution of photolithographic photoresist patterning, as describedbelow.

First, reflections cause the light to pass through the photoresist atleast twice, rather than only once. In other words, light first passesthrough the photoresist to reach the surface of the underlyingsubstrate. Then, light is reflected from the surface of the underlyingsubstrate and passes back through the photoresist layer a second time.The chemical structure of the photoresist changes differently when lightpasses through the photoresist more than once, rather than when lightpasses through the photoresist only once. A portion of the light,already reflected from the surface of the underlying substrate, can alsoreflect again from the surface of the photoresist, passing back throughthe photoresist yet again. In fact, standing light waves can result inthe photoresist from superpositioning of incident and reflected lightrays. This overexposure problem is sometimes referred to as the "swingeffect."

Even more problematic, the reflections of the light are not necessarilyperpendicular. Light reflects angularly from the surface of theunderlying substrate, even if the light is incident exactlyperpendicular to the surface of the substrate. This results from thediffractive nature of light (i.e., light bends). Off-angle reflectionsreduce the sharpness of the resulting photoresist pattern. A portion ofthe light reflected obliquely from the surface of the underlyingsubstrate can also be again reflected obliquely from the surface of thephotoresist. As a result of such angular reflections, the light cantravel well outside those photoresist regions underlying thetransmissive portions of the photolithographic mask. This potentiallycauses photoresist exposure well outside those photoresist regionsunderlying transmissive portions of the photolithographic mask. Thisproblem, which is sometimes referred to as "notching," results in a lesssharply defmed photoresist pattern that limits the density of structuresformed on the integrated circuit. There is a need to overcome thesephotolithographic limitations to obtain the benefits of high resolutionphotolithography and high density integrated circuits.

SUMMARY OF THE INVENTION

The present invention provides, among other things, an antireflectivecoating (ARC), such as for use in integrated circuit (IC)photolithography. In one embodiment, the invention provides anantireflective structure that includes a first layer formed on thesubstrate. A second layer is formed on the first layer. The second layerhas a working surface for receiving a photoresist layer formedthereupon. An optical impedance of the second layer, at an interfacebetween the first and second layers, is approximately equal to anoptical impedance of the first layer. The optical impedance of thesecond layer increases in a direction away from the interface betweenthe first and second layers. The optical impedance at the workingsurface of the second layer is approximately equal to the opticalimpedance of at least a portion of the photoresist layer.

In another embodiment, the present invention provides, among otherthings, an integrated circuit. The integrated circuit includes a firstlayer, having a first optical impedance, formed on a substrate. A secondlayer, having a second optical impedance, is formed on the first layer.The second layer has a gradient optical impedance. A photoresist layer,having a photoresist optical impedance, is formed on the second layer.

In another embodiment, the present invention provides, among otherthings, a gradient antireflective coating for an integrated circuit. Thecoating includes an optically absorptive first layer formed on anintegrated circuit substrate. A second layer is formed between the firstlayer and a photoresist layer. The second layer has an optical impedanceapproximately equal to an optical impedance of the first layer. Theoptical impedance of the second layer increases in a direction away fromthe interface between the first and second layers.

In another embodiment, the present invention provides, among otherthings, an antireflective structure that includes a first layer formedon the substrate. A second layer is formed on the first layer. Thesecond layer has a working surface for receiving a photoresist layerformed thereupon. A composition of the second layer, at an interfacebetween the first and second layers, is approximately equal to acomposition of the first layer, at an interface between the first andsecond layers. The composition of the second layer changes in adirection away from the interface between the first and second layers.

Another aspect of the present invention provides a method. A first layeris formed on an integrated circuit substrate. A second layer is formedon the first layer. A composition of at least a portion of the secondlayer is graded in a direction away from an interface between the firstand second layers. A photoresist layer is formed on the second layer.The substrate is exposed to incident light. Substantially all of theincident light that reaches the first layer is absorbed in the firstlayer.

In one embodiment, by way of example, but not by way of limitation, thepresent invention provides a method of forming an antireflective coatingon a substrate. A silicon carbide first layer is formed on thesubstrate. A substantially continuously graded composition siliconoxycarbide second layer is formed on the first layer. A portion of thesecond layer that is adjacent to the first layer has a materialcomposition that approaches that of silicon carbide.

Thus, the present invention provides, among other things, anantireflective coating (ARC), such as for use in integrated circuitphotolithography. The antireflective coating is interposed between aphotoresist layer and an underlying substrate. The present inventionmatches an optical impedance at the interface between the antireflectivecoating and the photoresist. The optical impedance decreases in theantireflective coating in a direction away from the interface betweenthe antireflective coating and the photoresist. Substantially allincident light received by the antireflective coating is absorbed in theantireflective coating. As a result, substantially no light reaches thepotentially highly reflective underlying substrate.

The present invention reduces swing effect and notching, therebyallowing higher resolution photolithography and resulting in higherdensity integrated circuits. The present invention includes embodimentsthat can be formed at low temperatures, making the antireflectivecoating particularly compatible with underlying aluminum or other metalinterconnection layers, which are particularly reflective and have a lowthermal budget. The present invention is also compatible withultraviolet (UV) and deep ultraviolet (DUV) photolithographic exposurewavelengths such as required for high resolution photolithography.Unlike certain prior art approaches, which only reduce reflections, thepresent invention is capable of completely eliminating reflections atthe interface between the photoresist and underlying antireflectivecoating. Unlike certain prior art approaches, which work only forparticular materials on an underlying substrate, the present inventionprovides an antireflective coating that works for substantially allunderlying substrate materials and also for substantially all overlyingphotoresist materials. Other advantages will also become apparent uponreading the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe substantially similar componentsthroughout the several views.

FIG. 1 is a cross-sectional view illustrating generally one embodimentof antireflective techniques according to the present invention.

FIG. 2 is a generalized graph of optical impedance (Z) vs. distance (x),where distance x is expressed in terms of a wavelength (λ) of incidentlight.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The terms wafer andsubstrate used in the following description include any structure havingan exposed surface with which to form the integrated circuit (IC)structure of the invention. The term substrate is understood to includesemiconductor wafers. The term substrate is also used to refer tosemiconductor structures during processing, and may include other layersthat have been fabricated thereupon. Both wafer and substrate includedoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures well known to one skilled in the art. The termconductor is understood to include semiconductors, and the terminsulator is defined to include any material that is less electricallyconductive than the materials referred to as conductors. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims, along with the full scope of equivalents to which such claimsare entitled.

The present invention provides, among other things, an antireflectivecoating (ARC), such as for use in IC photolithography. Theantireflective coating is interposed between a photoresist layer and anunderlying substrate. One aspect of the present invention matches anoptical impedance at the interface between the antireflective coatingand the photoresist. The optical impedance decreases in theantireflective coating in a direction away from the interface betweenthe antireflective coating and the photoresist. Substantially allincident light received by the antireflective coating is absorbed in theantireflective coating. As a result, substantially no light reaches thepotentially highly reflective underlying substrate.

The present invention reduces swing effect and notching, therebyallowing higher resolution photolithography and higher densityintegrated circuits. The present invention includes embodiments that canbe formed at low temperatures, making the antireflective coatingparticularly compatible with underlying aluminum or other metalinterconnection layers, which are particularly reflective and have a lowthermal budget. The present invention is also compatible with theultraviolet (UV) and deep ultraviolet (DUV) photolithographic exposurewavelengths required for high resolution photolithography. Moreover, thepresent invention also includes X-ray lithography, ion-beam lithography,and other lithographic techniques. Unlike certain prior art approaches,which only reduce reflections, the present invention is capable ofcompletely eliminating reflections at the interface between thephotoresist and underlying antireflective coating. Also unlike certainprior art approaches, which work only for particular underlyingsubstrate materials, the present invention provides an antireflectivecoating that works for substantially all underlying substrate materialsand overlying photoresist materials. Other advantages will also becomeapparent upon reading the following detailed description of theinvention.

FIG. 1 is a cross-sectional view illustrating generally, by way ofexample, but not by way of limitation, one embodiment of antireflectivetechniques according to the present invention. In FIG. 1, substrate 100includes any wafer or substrate, including any topology or structurespreviously fabricated thereupon, such as transistors or other circuitelements, interconnection lines (e.g., aluminum, copper, refractorymetals, or other conductive material), insulators for isolating circuitelements or interconnection lines, semiconductor structures, or anyother integrated circuit component. First layer 105 and second layer 110together form a backside antireflective coating (ARC), also referred toas an antireflective layer (ARL), which is interposed between substrate100 and photoresist layer 115. First and second layers, 105 and 110, areeither fabricated together in a single process step, or arealternatively separately formed during separate processing steps.Photoresist layer 115 is formed on a working surface 110A of secondlayer 110. Photoresist layer 115 is selectively exposed to incidentlight 120 using a photolithographic mask 125 overlying photoresist layer115. Photoresist layer 115 includes a pattern of transmissive (clear)portions 125A and absorptive (opaque) portions 125B, which arecharacterized as transmissive or absorptive for a particular wavelengthof the incident light 120.

First and second layers 105 and 110, respectively, minimize reflectionsof incident light 120 from substrate 100 back into photoresist layer115. Such reflections result in reduced-definition exposure of thephotoresist layer 115 (e.g., swing effect and notching), as describedabove. Higher density ICs require improved photolithographic resolution.Improved photolithographic resolution, in turn, requires shorterwavelengths of incident light 120 in order to limit diffractive effects.However, shorter wavelengths of incident light 120, such as ultraviolet(UV) and deep ultraviolet (DUV) wavelengths, typically result inincreased reflection from the working surface 100A of underlyingsubstrate 100 when prior art photolithographic techniques are used.Moreover, certain substrate materials, such as aluminum and otherinterconnection conductors, are highly reflective to incident UV and DUVlight.

According to one aspect of the present invention, as described below,first and second layers 105 and 110, respectively, minimize reflectionof incident light 120 back into the photoresist layer 115 from thesurface 110A of second layer 110. Moreover, incident light 120 that istransmitted through second layer 110, such that it reaches first layer105, is substantially absorbed in first layer 105. As a result, incidentlight 120 does not reach the surface 100A of reflective substrate 100,and is not reflected therefrom.

According to another aspect of the present invention, reflections fromsurface 110A of second layer 110 are minimized by matching, at surface110A, an optical impedance Z₁₁₀ of second layer 110, to a respectiveoptical impedance Z₁₁₅ of photoresist layer 115. A magnitude ofreflection coefficient K₁₁₀ A at surface 110A is illustrated byEquation 1. ##EQU1##

In Equation 1, K₁₁₀ A is the reflection coefficient at surface 110A,Z₁₁₀ is the optical impedance of second layer 110 at surface 110A, andZ₁₁₅ is the optical impedance of photoresist layer 115 adjacent tosurface 110A. According to one aspect of the invention, the opticalimpedance Z₁₁₀ of second layer 110 is complex-valued (i.e., having bothreal and imaginary parts, Z=R+j X), allowing easier matching to theoptical impedance Z₁₁₅ of photoresist layer 115, which is typically alsocomplex-valued for common photoresist layer 115 materials. As thenumerator of Equation 1 indicates, better matching of Z₁₁₀ and Z₁₁₅ atsurface 110A allows minimization of the magnitude of the reflectioncoefficient K₁₁₀ A at surface 110A. This minimizes reflections ofincident light 120 at surface 110A that cause swing effect and notching.

In one example of the present invention, each of the real and imaginaryparts of Z₁₁₀ and Z₁₁₅ are matched to respective real and imaginaryparts of the other of Z₁₁₀ and Z₁₁₅ at surface 110A. In another exampleof the present invention, the magnitudes of the complex-valued opticalimpedances Z₁₁₀ and Z₁₁₅ are matched to each other at the surface 110A.Alternatively, only the real parts of the optical impedances Z₁₁₀ andZ₁₁₅ (i.e., the optical absorptivities) are matched to each other atsurface 110A.

According to yet another aspect of the present invention, reflectionsfrom interface 105A between first layer 105 and second layer 110 areminimized by matching, at interface 105A, an optical impedance Z₁₀₅ offirst layer 105 to the optical impedance Z₁₁₀ of second layer 110. Amagnitude of reflection coefficient K₁₀₅ at interface 105A isillustrated by Equation 2. ##EQU2##

In Equation 2, K₁₀₅ is the reflection coefficient at interface 105A,Z₁₀₅ is the optical impedance of first layer 105 at interface 105A, andZ₁₁₀ is the optical impedance of second layer 110 at interface 105A.According to one aspect of the present invention, the optical impedanceZ₁₁₀ of second layer 110 is complex-valued, as described above.According to another aspect of the present invention, the opticalimpedances Z₁₀₅ and Z₁₁₀ are matched at interface 105A, as describedabove. As the numerator of Equation 2 indicates, better matching of Z₁₀₅and Z₁₁₀ at interface 105A minimizes the magnitude of the reflectioncoefficient K₁₀₅ A at interface 105A. This minimizes reflections ofincident light 120 from interface 105A.

In one example of the present invention, each of the real and imaginaryparts of Z₁₀₅ and Z₁₁₀ are matched to respective real and imaginaryparts of the other of Z₁₀₅ and Z₁₁₀ at interface 105A. In anotherexample of the present invention, the magnitudes of Z₁₀₅ and Z₁₁₀ arematched to each other at interface 105A. Alternatively, only the realparts of Z₁₀₅ and Z ₁₀ (i.e., the optical absorptivities) are matched toeach other at interface 105A. In a further embodiment, by way ofexample, but not by way of limitation, the optical impedance Z₁₁₀ (e.g.,its complex value, its magnitude, or at least one of the real andimaginary parts) of second layer 110 is substantially continuouslygraded between the value of Z ₁₁₀ at interface 105A and the value ofZ₁₁₀ at surface 110A.

In one embodiment, for example, a material composition of first layer105 includes silicon carbide (SiC), which is strongly absorptive to UVand DUV wavelengths of incident light 120. According to one aspect ofthis embodiment of the invention, the SiC first layer 105 isapproximately stoichiometric. According to another aspect of thisembodiment of the invention, the SiC first layer 105 isnonstoichiometric such as, by way of example, but not by way oflimitation, having a composition approximately between Si₀.65 C₀.35 andSi₀.3 C₀.7.

In one embodiment, second layer 110 includes silicon oxycarbide (SiOC)having a graded material composition (also referred to as a gradientmaterial composition). In another embodiment, by way of example, but notby way of limitation, second layer 110 includes SiOC having asubstantially continuously graded material composition. For example, atinterface 105A, the material composition of second layer 110 isapproximately equal to the material composition of SiC first layer 105.As a result, no significant discontinuity in optical impedance exists atinterface 105A, thereby substantially eliminating reflections ofincident light 120 from interface 105A. Similarly, at surface 110A, thematerial composition of SiOC second layer 110 is selected to match anoptical impedance Z₁₁₀ of second layer 110 to the optical impedance Z₁₁₅in photoresist layer 115. Thus, between interface 105A and surface 110A,the optical impedance Z₁₁₀ is graded. In one embodiment, the gradientoptical impedance Z₁₁₀ is obtained by grading the material compositionof second layer 110.

As described below, a complete range of optical impedances Z₁₁₀ isobtainable in second layer 110. Moreover, the complete range of opticalimpedances Z₁₁₀ is repeated in second layer 110 at intervals of 1/4wavelength of the incident light 120. This enables the present inventionto accurately match the optical impedance Z₁₁₀ of second layer 110 tothe optical impedance Z₁₁₅ in photoresist layer 115 at interface 110Afor a photoresist layer 115 of any material and optical impedance Z₁₁₅.

According to one aspect of the invention, the thickness of first layer105 is selected such that any incident light 120 received by first layer105 is substantially absorbed or extinguished therein. In oneembodiment, for example, the present invention provides a stronglyabsorptive SiC first layer 105, having a thickness of greater than orequal to approximately between 100 angstroms to 200 angstroms, thatsubstantially absorbs all of the UV or DUV incident light 120 thatreaches first layer 105. This prevents any reflections from surface 100Aof underlying substrate 100, which is potentially highly reflective. Theexact thickness of first layer 105 varies according to, among otherthings, the wavelength of the incident light 120, the degree of opticalabsorption expected in overlying second layer 110, the degree of opticalabsorption expected in first layer 105, and the desired degree to whichthe incident light 120 is extinguished in first layer 105. For example,if a lesser degree of antireflective protection is required, then lessoptical absorption in first layer 105 is required, and stronglyabsorptive SiC first layer 105 can have a lesser thickness (e.g., 10angstroms, 50 angstroms, etc.).

According to another aspect of the invention, the thickness of secondlayer 110 is selected for matching optical impedance Z₁₁₀ (to Z₁₁₅ atsurface 110A and to Z₁₀₅ at interface 105A), and for obtaining a desireddegree of grading of the material composition of second layer 110between surface 110A and interface 105A. In one embodiment, for example,the thickness of second layer 110 is less than or equal to 1/4 of thewavelength of the incident light 120, since a complete range of opticalimpedances Z₁₁₀ is obtainable within that distance. Moreover,sign-changes in the optical impedance Z₁₁₀ exist at 1/4 wavelengthintervals, and discontinuities in the optical impedance Z₁₁₀ exist at1/2 wavelength intervals, as illustrated in FIG. 2, which is ageneralized graph of optical impedance (Z) vs. distance (x), wheredistance (x) is expressed in terms of the wavelength (λ) of the incidentlight 120.

Another embodiment of the present invention, however, uses a thicknessof second layer 110 that exceeds 1/4 wavelength of the incident light120 such as, for example, if process constraints make it difficult toobtain the desired compositional rate of change of SiOC second layer 110within a distance of 1/4 wavelength of the incident light 120. Since acomplete range of magnitudes of optical impedances Z₁₁₀ is available ineach 1/4 wavelength interval, the thickness of second layer 110 can alsoexceed 1/4 wavelength of incident light 120, as seen in FIG. 2, whilestill providing the desired impedance matching properties at each ofinterface 105A and surface 110A.

In an alternative embodiment, for example, first layer 105 includespolycrystalline silicon (Si), which is also strongly absorptive to Uvand DUV wavelengths of incident light 120. In this embodiment, secondlayer 110 includes silicon oxycarbide (SiOC) having a graded materialcomposition, such as a substantially continuously graded SiOC materialcomposition. At interface 105A, the material composition of second layer110 is approximately equal to the material composition ofpolycrystalline Si first layer 105. As a result, no significantdiscontinuity in optical impedance exists at interface 105A, therebysubstantially eliminating reflections of incident light 120 frominterface 105A. Similarly, at surface 110A, the material composition ofSiOC second layer 110 is selected to match an optical impedance Z₁₁₀ ofsecond layer 110 to the optical impedance Z₁₁₅ of photoresist layer 115,as described above. According to another aspect of the invention, thematerial composition of SiOC second layer 110 is graded (e.g.,substantially continuously) from that of polycrystalline Si, atinterface 105A, to the above-described desired SiOC composition atsurface 110A.

Though the above examples illustrate particular material compositions ofeach of first layer 105 and second layer 110, the present invention isunderstood to include, among other things, the use of any materialsproviding a strongly absorptive or low optical impedance first layer 105and substantially continuously graded second layer 110 materialcomposition or optical impedance Z₁₁₀. Moreover, first layer 105 andsecond layer 110 can be formed together, in a single process step, orseparately, in more than one process step.

Process

According to one aspect of the invention, first layer 105 includesamorphous or polycrystalline silicon (Si) formed by chemical vapordeposition (CVD). According to another aspect of the invention, firstlayer 105 includes stoichiometric or nonstoichiometric silicon carbide(SiC). One embodiment of forming SiC first layer 105 is described in theForbes et al. U.S. patent application Ser. No. 08/903,486, entitled"SILICON CARBIDE GATE TRANSISTOR AND FABRICATION PROCESS", filed on Jul.29, 1997, which disclosure is assigned to the assignee of the presentpatent application, and which disclosure is incorporated herein byreference.

In one embodiment, for example, SiC first layer 105 is deposited usinglow-pressure chemical vapor deposition (LPCVD). The LPCVD process useseither a hot-wall reactor or a cold-wall reactor with a reactive gas,such as a mixture of Si(CH₃)₄ and Ar. Examples of such processes aredisclosed in an article by Y. Yamaguchi et al., entitled "Properties ofHeteroepitaxial 3C-SiC Films Grown by LPCVD", in the 8th InternationalConference on Solid-State Sensors and Actuators and Eurosensors IX,Digest of Technical Papers, page 3. vol. (934+1030+85), pages 190-3,Vol. 2, 1995, and in an article by M. Andrieux, et al., entitled"Interface and Adhesion of PECVD SiC Based Films on Metals", insupplement Le Vide Science. Technique et Applications. (Francel), No.279, pages 212-214, 1996. However, SiC first layer 105 can also bedeposited using other techniques such as, for example, enhanced CVDtechniques including low pressure rapid thermal chemical vapordeposition (LP-RTCVD), or by decomposition of hexamethyl disalene usingArF excimer laser irradiation, or by low temperature molecular beamepitaxy (MBE). Other examples of forming SiC first layer 105 includereactive magnetron sputtering, DC plasma discharge, ion-beam assisteddeposition, ion-beam synthesis of amorphous SiC films, lasercrystallization of amorphous SiC, laser reactive ablation deposition,and epitaxial growth by vacuum anneal.

In another embodiment, SiC first layer 105 is formed by plasma-assistedCVD using silane and methane. See e.g., M. M. Rahman et al.,"Preparation and Electrical Properties of An Amorphous SiC/CrystallineSi p⁺ n Heterostructure," Jpn. J. Appl. Phys., Vol. 26, pp. 515-524,(1984), which disclosure is incorporated herein by reference. Forexample, substrate 100 is heated to a temperature of approximately 250degrees Celsius. Silane (SiH₄) and methane (CH₄) gases are introduced inthe presence of an RF (13.56 MHz) plasma at a power that isapproximately between 10 Watts and 100 Watts. At an RF power ofapproximately 10 Watts, SiC first layer 105 is formed at a filmdeposition rate that ranges approximately between 12 angstroms perminute and 180 angstroms per minute when the silane and methane gasesare provided at pressures that range approximately from 0.1 Torr. to 0.6Torr. The exact material composition of the SiC first layer 105 is finetuned by varying the RF power at a particular gas pressure. In oneembodiment, for example, a gas pressure of 0.4 Torr and RF plasma powerof approximately 20 Watts forms an SiC first layer 105 having a materialcomposition of approximately Si₀.65 C₀.35. In another embodiment, forexample, a gas pressure of 0.4 Torr and an RF plasma power ofapproximately 100 Watts forms an SiC first layer 105 having a materialcomposition of approximately Si₀.3 C₀.7. The resulting SiC first layer105 has a thickness that is approximately between 100 angstroms and 200angstroms.

According to another aspect of the invention, second layer 110 includessilicon oxycarbide (SiOC) having a graded material composition, such asa substantially continuously graded SiOC material composition. In oneembodiment, by way of example, but not by way of limitation, SiOC secondlayer 110 is formed by high temperature deposition, such as by pyrolysisof silicone polymer resins. See e.g., G. M. Renlund et al., "SiliconOxycarbide Glasses. I. Preparation and Chemistry," J. MaterialsResearch, Vol. 6, No. 12, pp. 2716-22, December 1991. Starting polymersinclude methyl trichlorosilane and dimethyl dichlorosilane. Heatingsubstrate 100 to a temperature of approximately between 700 degreesCelsius and 1000 degrees Celsius produces resulting amorphous SiOCsecond layer 110. SiOC second layer 110 remains amorphous attemperatures below approximately 1500 degrees Celsius, above which ittends to become crystalline SiC.

In another embodiment, by way of example, but not by way of limitation,SiOC second layer 110 is formed by pyrolytic conversion ofpoly-silsesquioxane copolymers. The silsesquioxane copolymers havevarying ratios of phenyl to methyl groups, such as by synthesis throughhydrolysis and condensation of phenyltrimethoxysilanes andmethyltrimethoxysilanes. The ratio of phenyl to methyl groups controlsthe resulting material composition of the SiOC second layer 110. TheSiOC second layer 110 is formed by pyrolysis at temperaturesapproximately between 1000 degrees Celsius and 1200 degrees Celsius.Heating beyond 1200 degrees Celsius diminishes the SiOC structure, andthe material becomes primarily amorphous silica, amorphous SiC, somesmall crystallites of SiC, and graphitic carbon.

In another embodiment, by way of example, but not by way of limitation,SiOC second layer 110 is formed at low temperatures. This isparticularly useful for forming an antireflective coating over asubstrate 100 that includes low thermal budget layers such as, forexample, aluminum or other metallization layers. Such layers cannotwithstand subsequent high temperature processing steps. SiOC secondlayer 110 is formed by plasma-assisted CVD. Substrate 100 is heated to atemperature of approximately 100 degrees Celsius. A mixture of silaneand methane gases, to which oxygen or nitrous oxide (N₂ O) is added, isintroduced in the presence of an RF (13.56 MHz) plasma at a power ofapproximately 120 Watts.

According to one aspect of the invention, the composition of the SiOCsecond layer 110 is changed by varying the ratio of nitrous oxide tomethane during formation of second layer 110. This provides a gradedmaterial composition SiOC second layer 110. Continuously varying theratio of nitrous oxide to methane during formation of second layer 110provides a substantially continuously graded SiOC material compositionof second layer 110.

By increasing the ratio of nitrous oxide to methane, the SiOCcomposition contains more oxygen and less carbon. Thus, in oneembodiment, the composition of SiOC second layer 110 is variedsubstantially continuously from SiC to SiO₂ by initially providing onlysilane and methane. Then, nitrous oxide is added to the silane andmethane, in an increasing nitrous oxide to methane ratio. Finally, onlysilane and nitrous oxide is provided, in one embodiment. In otherembodiments, the exact final ratio of the silane, methane, and nitrousoxide constituent gases depends on the particular desired composition ofSiOC second layer 110 at surface 110A, as described above.

According to another aspect of the invention, the composition of theSiOC second layer 110 is changed by varying the ratio of nitrous oxideto silane, such as to obtain a substantially continuously gradedcomposition SiOC second layer 110. By increasing the ratio of nitrousoxide to silane, the SiOC composition contains more oxygen and lesssilicon. Thus, in one embodiment, the composition of SiOC second layer110 is varied substantially continuously from Si to SiOC by initiallyproviding only silane. Then, nitrous oxide and methane are added to thesilane, in an increasing ratio of nitrous oxide and methane to silane.The exact final ratio of the silane, methane, and nitrous oxideconstituent gases depends on the particular desired composition of SiOCsecond layer 110 at surface 110A.

Conclusion

As described above, the present invention provides, among other things,an antireflective coating (ARC), such as for use in integrated circuitphotolithography. The antireflective coating is interposed between aphotoresist layer and an underlying substrate. One aspect of the presentinvention matches an optical impedance at the interface between theantireflective coating and the photoresist. The optical impedancedecreases in the antireflective coating in a direction away from theinterface between the antireflective coating and the photoresist.Substantially all incident light is that reaches the antireflectivecoating is absorbed in the antireflective coating. As a result,substantially no light reaches the potentially highly reflectiveunderlying substrate.

The present invention reduces swing effect and notching, therebyallowing higher resolution photolithography and higher densityintegrated circuits. The present invention includes embodiments that canbe formed at low temperatures, making the antireflective coatingparticularly compatible with underlying aluminum or metalinterconnection layers, which are particularly reflective and have a lowthermal budget. The present invention is also compatible with theultraviolet (UV) and deep ultraviolet (DLUV) photolithographic exposurewavelengths required for high resolution photolithography. Unlikecertain prior art approaches, which only reduce reflections, the presentinvention is capable of completely eliminating reflections at theinterface between the photoresist and underlying antireflective coating.Unlike certain prior art approaches, which work only for particularmaterials on an underlying substrate, the present invention provides anantireflective coating that works for substantially all underlyingsubstrate materials and overlying photoresist materials. Otheradvantages are also described in the above detailed description of theinvention.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A method comprising:forming either a silicon orsilicon carbide layer on an integrated circuit substrate; coating thesilicon or silicon carbide layer with a graded composition siliconoxycarbide layer having an optical impedance approximately equal to anoptical impedance of the silicon or silicon carbide layer, and theoptical impedance of the silicon oxycarbide layer increasing in adirection away from the interface between the silicon or silicon carbideand silicon oxycarbide layers; forming a photoresist layer on thesilicon oxycarbide layer; exposing the substrate to incident lighthaving a wavelength; and absorbing, in the silicon or silicon carbidelayer, substantially all of any of the incident light that is receivedby the silicon or silicon carbide layer.
 2. The method of claim 1, inwhich forming the second layer on the first layer includes substantiallycontinuously grading the composition of at least a portion of the secondlayer in a direction away from the interface between the first andsecond layers.
 3. The method of claim 1, in which forming the secondlayer includes approximately matching an optical impedance of the firstand second layers at an interface between the first and second layers.4. The method of claim 1, in which forming the second layer includesapproximately matching an optical impedance between the second andphotoresist layers at an interface between the second and photoresistlayers.
 5. The method of claim 1, in which the second layer includesforming silicon oxycarbide having a composition that includes moreoxygen and less carbon in a direction away from the interface betweenthe first and second layers.
 6. The method of claim 1, in which thesecond layer includes forming silicon oxycarbide having a compositionthat includes more oxygen and less silicon in a direction away from theinterface between the first and second layers.
 7. A method of forming anantireflective coating on a substrate, the method comprising:forming asilicon carbide first layer on the substrate; and forming a gradedcomposition silicon oxycarbide second layer on the first layer,including forming a portion of the second layer that is adjacent to thefirst layer and that has a material composition that approaches that ofsilicon carbide.
 8. The method of claim 7, wherein forming the firstlayer includes:heating the substrate; and introducing silane and methanegases in the presence of an RF plasma.
 9. The method of claim 8, furthercomprising adjusting a power of the RF plasma to obtain a desiredsilicon carbide first layer material composition.
 10. The method ofclaim 8, further comprising adjusting a gas pressure of at least one ofthe silane and methane gases to obtain a desired silicon carbide firstlayer material composition.
 11. The method of claim 8, wherein heatingthe substrate includes heating at a temperature of approximately 250degrees Celsius, and introducing silane and methane gases includesapplying an RF plasma at a power that is approximately between 10 Wattsand 100 Watts, and further comprising providing a silane and methane gaspressure of approximately between 0.1 Torr. to 0.6 Torr.
 12. The methodof claim 7, wherein forming the second layer includesheating thesubstrate; and introducing silane, methane, and nitrous oxide gases inthe presence of an RF plasma.
 13. The method of claim 12, whereinheating the substrate includes heating at a temperature of approximately100 degrees Celsius.
 14. The method of claim 12, wherein introducing thesilane, methane, and nitrous oxide gases includes varying a combinationof the silane, methane, and nitrous oxide gases to obtain the gradedcomposition silicon oxycarbide second layer.
 15. The method of claim 12,wherein introducing the silane, methane, and nitrous oxide gasesincludes substantially continuously varying a combination of the silane,methane, and nitrous oxide gases to obtain a substantially continuouslygraded composition silicon oxycarbide second layer.
 16. The method ofclaim 7, wherein forming the second layer includes pyrolyzingpoly-silsesquioxane copolymers.
 17. The method of claim 16, whereinpyrolyzing poly-silsesquioxane copolymers includes providingpoly-silsesquioxane copolymers of varying ratios of phenyl to methylgroups to obtain a graded material composition silicon oxycarbide secondlayer.